1. Field of the Invention
The present invention relates to a catalyst state detector for an exhaust gas purifying catalyst which purifies exhaust gases emitted from an internal combustion engine, and more particularly, to a catalyst state detector for an exhaust gas purifying catalyst which purifies exhaust gases using an adsorbent for adsorbing hydrocarbons in exhaust gases.
2. Description of the Prior Art
Generally, in an exhaust system of a gasoline engine or the like, a three-way catalyst is arranged in an intermediate portion of an exhaust pipe in order to purify harmful substances (hydrocarbons, carbon monoxide and nitrogen compounds) in exhaust gases, the amount of which cannot be reduced sufficiently by engine modifications or EGR (exhaust gas recirculation). The three-way catalyst is heated by exhaust gases or by using additional means, and activated at temperatures equal to or higher than a predetermined temperature (300xc2x0 C., for example), thereby purifying harmful substances flowing through the exhaust pipe by oxidation-reduction catalyst actions thereof. However, for approximately 30 to 40 seconds after the cold start of the engine, the temperature of the three-way catalyst is lower than the predetermined temperature, and the catalyst remains inactive, so that among the harmful substance, particularly hydrocarbons are emitted from the engine as they are as unburned combustible components. Therefore, in order to prevent emission of hydrocarbons in the air, there has been proposed an engine which incorporates not only the three-way catalyst but also an adsorbent, which is capable of adsorbing hydrocarbons, in an exhaust pipe of the engine (see, for example, Laid-open Japanese Patent Application No. 10-153112).
Such an adsorbent is arranged in a bypass exhaust passage branched from a main exhaust passage in which a three-way catalyst is arranged in exhaust pipes. When the three-way catalyst is not activated, for example, upon starting the engine, exhaust gases are introduced into the bypass exhaust passage such that hydrocarbons in the exhaust gases are adsorbed by the adsorbent. On the other hand, after the three-way catalyst is activated by worm-up of the engine, the exhaust gases are introduced into the main exhaust passage such that the exhaust gases are purified by the three-way catalyst. The adsorbent has, for example, zeolite on its surface, so that when the exhaust gases are passing through the bypass exhaust passage, molecules of hydrocarbons are caused to enter small pores of the zeolite, and the hydrocarbons are adsorbed. Also, when such an adsorbent is heated to a temperature equal to or higher than a predetermined temperature (for example, 100 to 250xc2x0 C.) by the exhaust gases, the adsorbent desorbs hydrocarbons once adsorbed thereby. Then, the desorbed hydrocarbons are recirculated to the engine through an EGR pipe or the like.
As described above, while adsorption and desorption of hydrocarbons are repeated in the adsorbent, the amount of undesorbed hydrocarbons remaining in the adsorbent may gradually increase or cause destruction of pores of the adsorbent during a long-term use of the adsorbent. As a result, the adsorbent is deteriorated, i.e., experiences a gradually deteriorated capability of adsorbing hydrocarbons. If the engine is repeatedly started in such a state, hydrocarbons unabsorbed by the adsorbent will be emitted into the air. For this reason, it is required to detect the deterioration of the adsorbent in order to carry out engine control for desorbing hydrocarbons (e.g., by elevating the temperature of the adsorbent) to solve the deterioration of the adsorbent, or to notify the driver of the deterioration of the adsorbent. Among such methods of detecting the deterioration of the adsorbent, there are, by way of example, (1) a method which relies on a hydrocarbon sensor for the detection, (2) a method which relies on temperature sensors for the detection, and so on.
According to the method (1) relying on a hydrocarbon sensor, the hydrocarbon sensor is arranged at a location downstream of the adsorbent in the bypath exhaust passage. Then, the concentrations of hydrocarbons in exhaust gases introduced into the bypass exhaust passage and passing through the adsorbent is directly detected by the hydrocarbon sensor. Then, the deterioration of the adsorbent is detected based on the result of the detection. Specifically, when the concentrations of hydrocarbons in the exhaust gases introduced into the bypass exhaust passage and passing at a location downstream of the adsorbent (hereinafter referred to as the xe2x80x9cpost-adsorption exhaust gasesxe2x80x9d in the disclosure), which is active in adsorbing hydrocarbons, exceed a predetermined value or a predetermined allowable range, it is determined that the adsorbent is no longer capable of sufficiently adsorbing hydrocarbons, i.e., in a deteriorated state. Conversely, when the concentrations of hydrocarbons in the post-adsorption exhaust gases are equal to or lower than the predetermined value or within the predetermined allowable range, it is determined that the adsorbent is still capable of properly adsorbing hydrocarbons, i.e., in an undeteriorated or normal state.
According to the method (2) relying on temperature sensors, on the other hand, the temperature sensors are arranged both at locations upstream and downstream of an adsorbent in a bypass exhaust passage, respectively, for example, as described in Laid-open Japanese Patent Application No. 6-229234. These sensors detect the temperatures of the exhaust gases at locations upstream and downstream of the adsorbent to calculate a time period during which moisture in the exhaust gases is in a dew point state (dew point time) based on the result of the detection. Then, the deterioration of the adsorbent is detected based on the resulting dew point time. More specifically, a predefined standard dew point time (standard dew point time) is calculated in accordance with the result of the detection by the upstream temperature sensor and an engine operating condition, and an actual dew point time (actual dew point time) is calculated based on the result of the detection by the downstream temperature sensor. Then, these dew point times are compared with each other to determine that the adsorbent is more deteriorated as the actual dew point time is shorter than the standard dew point time, and conversely to determine that the adsorbent is less deteriorated as the actual dew point time is longer than the standard dew point time.
The foregoing detecting methods suffer from the following problems, respectively. In the method (1) relying on a hydrocarbon sensor, a generally employed hydrocarbon sensor is limited to detection of the concentration of a particular hydrocarbon (one kind of hydrocarbon having a predetermined number of carbons), so that it is difficult to accurately detect as a whole the concentrations of various hydrocarbons respectively having different numbers of carbons by such a hydrocarbon sensor. It is further difficult to correctly detect the deterioration of the adsorbent based on the result of the detection. In addition, the hydrocarbon sensor itself is quite expensive as compared with other sensors.
In the method (2) relying on temperature sensors, on the other hand, the deterioration of the adsorbent is indirectly detected by detecting temperatures of exhaust gases at two different locations, causing a susceptibility to an error in the detection of deterioration and a low detection accuracy. In addition, temperature sensors are expensive as is the case of the hydrocarbon sensor, and the use of two temperature sensors further results in a higher cost of the exhaust system as a whole.
The present invention has been made to solve the problems as mentioned above, and its object is to provide a catalyst state detector for an exhaust gas purifying catalyst, which is capable of accurately detecting a state of an adsorbent of the exhaust gas purifying catalyst for adsorbing hydrocarbons, including deterioration, and which can be manufactured at a low price.
To achieve the above object, the present invention provides a catalyst state detector for detecting a state of an adsorbent capable of adsorbing hydrocarbons and moisture in exhaust gases of an internal combustion engine. The adsorbent is contained in an exhaust gas purifying catalyst arranged at an intermediate portion of an exhaust pipe in the internal combustion engine. The catalyst state detector includes a downstream humidity sensor arranged at a location downstream of the adsorbent in the exhaust pipe for detecting a humidity of the exhaust gases, and adsorbent state detecting means for detecting a state of the adsorbent in accordance with a result of a detection made by the downstream humidity sensor.
According to this catalyst state detector for an exhaust gas purifying catalyst, the humidity of exhaust gases downstream of the adsorbent (hereinafter referred to as the xe2x80x9cdownstream humidityxe2x80x9d) is detected by the downstream humidity sensor arranged at a location downstream of the adsorbent of the exhaust gas purifying catalyst in the exhaust pipe, and the state of the adsorbent is detected by the adsorbent state detecting means in accordance with the result of the detection. Since the capabilities of the adsorbent for adsorbing hydrocarbons and moisture are in a proportional relationship to each other, the downstream humidity has a high correlation to the amount of hydrocarbons actually adsorbed in the adsorbent. It is therefore possible to accurately detect the state of the adsorbent including adsorption and desorption of hydrocarbons in the adsorbent, deterioration of the adsorbent, and so on by detecting the downstream humidity. Also, since the catalyst state detector employs the humidity sensor cheaper than a hydrocarbon sensor or a temperature sensor, the detector itself can be manufactured at a lower cost.
Preferably, the catalyst state detector further comprises upstream humidity detecting means for detecting a humidity of the exhaust gases at a location upstream of the adsorbent, wherein the adsorbent state detecting means detects the state of the adsorbent in accordance with the upstream humidity detected by the upstream humidity detecting means.
According to this preferred embodiment of the catalyst state detector, by detecting the humidity of the exhaust gases at a location upstream of the adsorbent (upstream humidity) by the upstream humidity detecting means, together with the downstream humidity, i.e., by detecting the humidities at locations upstream and downstream of the adsorbent, the state of the adsorbent can be more correctly detected. The detection of the upstream humidity by the upstream humidity detecting means may include estimation of the upstream humidity in addition to the actual detection thereof.
Preferably, in the catalyst state detector, the upstream humidity detecting means estimates the upstream humidity based on the result of the detection made by the downstream humidity sensor.
According to this preferred embodiment of the catalyst state detector, since the upstream humidity is estimated based on the result of the detection made by the downstream humidity sensor, the state of the adsorbent can be detected using only a single humidity sensor (downstream humidity sensor), so that the detector can be manufactured at a lower cost.
Preferably, in the catalyst state detector, the upstream humidity detecting means is arranged at a location upstream of the adsorbent in the exhaust pipe, and comprises an upstream humidity sensor for detecting the upstream humidity.
According to this preferred embodiment of the catalyst state detector, by detecting the actual upstream humidity by the upstream humidity sensor, the state of the adsorbent can be more accurately detected.
Preferably, the catalyst state detector further includes response delay compensating means for compensating at least one of the downstream humidity sensor and the upstream humidity sensor for a response delay.
According to this preferred embodiment of the catalyst state detector, at least one of the downstream humidity sensor and the upstream humidity sensor is compensated by the response delay compensating means for a response delay, thereby making it possible to correctly detect the state of adsorbent even with a humidity sensor which exhibits a low responsibility, while properly compensating such a humidity sensor for the response delay.
Preferably, in the catalyst state detector, each of the downstream humidity sensor and the upstream humidity sensor includes a sensor element exposed to the exhaust gases for detecting the humidity of the exhaust gases, wherein the detector further includes a heater for heating the sensor element of at least one of the downstream humidity sensor and the upstream humidity sensor, operating state detecting means for detecting an operating state of the internal combustion engine, and heater control means for controlling an operation of the heater in accordance with the operating state detected by the operating state detecting means.
According to this preferred embodiment of the catalyst state detector, the heater control means operates the heater in accordance with the operating state of the internal combustion engine such that the heater heats the sensor element of at least one of the downstream humidity sensor and the upstream humidity sensor, thereby making it possible to bring the heated sensor element into a state suitable for detecting the humidity. For example, since the sensor element is susceptible to the inability of correctly detecting the humidity due to dew condensation or coke deposition, the sensor element may be heated when the internal combustion engine is likely to produce the dew condensation and coke deposition to avoid such inconveniences, thereby making it possible to correctly detect the humidity.
Preferably, in the catalyst state detector, the adsorbent state detecting means includes an adsorbent deterioration detecting means for detecting deterioration of the adsorbent as a state of the adsorbent.
According to this preferred embodiment of the catalyst state detector, the adsorbent deterioration detecting means is used to detect deterioration of the adsorbent in accordance with the result of detection made by the downstream humidity sensor, i.e., the downstream humidity. In other words, by detecting the humidity of exhaust gases after hydrocarbons and moisture therein have been adsorbed by the adsorbent (post-adsorption exhaust gases), it is possible to detect deteriorated capabilities of adsorbing hydrocarbons and moisture in the adsorbent, i.e., the deterioration of the absorbent. For example, when the adsorbent is not deteriorated, much of moisture in the post-adsorption exhaust gases is adsorbed by the adsorbent, so that the humidity in the post-adsorption exhaust gases is detected to be low. From this result of the detection, it is possible to estimate that the adsorbent is capable of satisfactorily adsorbing hydrocarbons as well as moisture, and therefore determine that the adsorbent is not deteriorated. On the other hand, when the adsorbent is deteriorated, moisture in the post-adsorption exhaust gases is not adsorbed so much, so that the humidity in the post-adsorption exhaust gases is detected to be high. From this result of the detection, it is possible to estimate that the adsorbent is no longer capable of sufficiently adsorbing hydrocarbons as well as moisture, and therefore determine that the adsorbent is deteriorated. In this way, the catalyst state detector according to this preferred embodiment can accurately detect the deterioration of the adsorbent by detecting the downstream humidity which has a high correlation to the deterioration of the adsorbent.
Preferably, in the catalyst state detector, the adsorbent deterioration detecting means detects the deterioration of the adsorbent based on a transition of a result of detection made by the downstream humidity sensor from a start of the internal combustion engine.
According to this preferred embodiment of the catalyst state detector, since the result of the detection by the downstream humidity sensor from the start of the internal combustion engine transitions in accordance with the degree of deterioration of the adsorbent, the deterioration of the adsorbent can be detected reversely based on this transition. As such a transition of the result of the detection made by the downstream humidity sensor, it is possible to use, for example, a time required for the humidity of the post-adsorption exhaust gases to reach a predetermined humidity from the start of the engine, a changing amount of the humidity of the post-adsorbent exhaust gases per unit time, the humidity of the post-adsorption exhaust gases after the lapse of a predetermined time from the start. By comparing these parameters with the counterparts derived by a new adsorbent, which have been previously measured, the deterioration of the adsorbent can be correctly detected based on the magnitude of the difference therebetween.
Preferably, the catalyst state detector further includes atmospheric state detecting means for detecting an atmospheric state, and deterioration detection execution determining means for determining whether or not the adsorbent deterioration detecting means should perform the deterioration detection based on results of detections by the atmospheric state detecting means and the downstream humidity sensor at the start of the internal combustion engine.
According to this preferred embodiment of the catalyst state detector, it is determined by the deterioration detection execution determining means whether or not the deterioration detection should be executed by the adsorbent deterioration detecting means based on the atmospheric state detecting means and the result of detection made by the downstream humidity sensor at the start of the internal combustion engine. In this event, assume, for example, that the saturated absolute humidity in the atmosphere at the start of the engine is detected (calculated) as the atmospheric state, and the adsorbent is determined to be deteriorated when the absolute humidity of the post-adsorption exhaust gases has become larger than the initially detected absolute humidity (initial absolute humidity) by a predetermined amount (the sum of the initial absolute humidity and the predetermined amount) until a predetermined time elapses from the start of the engine. When the humidity in the atmosphere is high on a rainy day or the like, the absolute humidity of the post-adsorption exhaust gases may have reached the saturated absolute humidity before the predetermined time elapses from the start of the engine. In this event, since the downstream humidity sensor indicates the value of the saturated absolute humidity afterwards, an erroneous determination could be made that the adsorbent is not deteriorated even though the adsorbent is actually deteriorated. Therefore, by preventing the execution of the deterioration detection for the adsorbent when the sum of the initial absolute humidity and the predetermined amount is equal to or more than the saturated absolute humidity, it is possible to avoid an erroneous determination as to the deterioration of the adsorbent.
Preferably, in the catalyst state detector, the adsorbent includes zeolite.
According to this preferred embodiment of the catalyst state detector, since the adsorbent includes zeolite, it is possible to provide an adsorbent which is highly heat resistant and also invulnerable to deterioration, as compared with, for example, silica gel, active carbon, or the like used as an adsorbent. In addition, such an adsorbent can properly adsorb hydrocarbons in exhaust gases at low temperatures at the start of the internal combustion engine. Also, the adsorbent comprised of zeolite can desorb hydrocarbons once adsorbed thereby at high temperatures without fail. Further, since zeolite exhibits a significantly high correlation of adsorption characteristics for hydrocarbons and moisture, it is possible to improve the accuracy in the detection of deterioration of the adsorbent by the catalyst state detector for an exhaust gas purifying catalyst which acts as a deterioration detector.